Interpretive Summary: Salmonella is one of the most common foodborne pathogenic bacteria and is frequently associated with swine. Bacteriophage are viruses that prey on bacteria and are commonly found in the gut of animals, but the incidence of these viruses are unknown. Phage have been proposed for use as a way to reduce Salmonella in food animals before the animals are slaughtered. However, for this to be a viable strategy, we must know how many phage are in the environment naturally. In this study, 600 commercial swine fecal samples were examined for bacteriophage. Generic phage were found in 48% of the fecal samples, but phage that killed the pathogen Salmonella Typhimurium were found in only 1% of the samples. While 7% of the swine feces contained a type of Salmonella, it appears that the phage have a pretty narrow activity spectrum. This means they must be specifically chosen to kill pathogens on a farm, or a cocktail of phage active against many serotypes must be used against Salmonella.

Technical Abstract:
Salmonella is one of the primary foodborne pathogens associated with swine production and represents a significant threat to human health. Bacteriophage are naturally-occurring viruses that prey on bacteria and have been suggested as a potential intervention strategy to reduce Salmonella in food animals on the farm and in the lairage period. If phage are to be used to improve food safety, then we must understand their incidence and natural ecology in the intestinal environment. Therefore, the current study was designed to determine the incidence of phage that are active against Salmonella spp. in the feces of commercial finishing swine in the United States. Fecal samples (n=60) were collected from each of ten commercial swine finishing operations. Samples were collected from ten randomly selected pens throughout each operation; the total number of fecal samples collected in this study was n=600. Salmonella spp. were found in 7.3% (44/600) of the fecal samples. Salmonella spp. were isolated from five farms, and the serotypes represented were Anatum, Copenhagen, Heidelberg, Johannesburg, Ohio, Schwarzengrund, and Typhimurium. Bacteriophages were isolated from fecal samples through 2 parallel methods: 1) initial enrichment in Salmonella Typhimurium, or 2) initial enrichment in E. coli B (an indicator strain), followed by direct spot-testing against Salmonella Typhimurium. Bacteriophages active against Salmonella Typhimurium were isolated from 1% (6/600) of the individual fecal samples when initially enriched in Salmonella Typhimurium, and E. coli B-killing phages were isolated from 48.3% (290/600) of the fecal samples, but only 2 of these phage were capable of killing Salmonella Typhimurium on secondary plating. Phages that killed E. coli F18, which is involved in post-weaning diarrhea, were found in 19.2% of the samples tested. Collectively, our results indicate that bacteriophage are widespread in commercial swine, but those capable of killing Salmonella Typhimurium may be present at relatively low population densities. These results indicate that phage (predator) populations may vary along with Salmonella (prey) populations and that phage could potentially be used as a food safety pathogen reduction strategy.